[fm]watch asked me this on irc, and i said to post the question here, because it's easier to answer here, and other people might be interested too.

anyway, here's a bit i lifted from a .qc file that i've already used to compile different skins into a model.

$texturegroup "skins"
{
{ "blackpaint.vmt" }
{ "greenpaint.vmt" }
{ "rustpaint.vmt" }
}

the first texture that is referenced, blackpaint.vmt is the texture that the reference.smd was wearing when i exported it from milkshape. for hl2 models, you should note that it's the .vmt extension that must be used. for hl1 models, it would be a .bmp extension.

greenpaint.vmt and rustpaint.vmt are the alternative skins.

if the model had been exported to .smd while wearing more than one texture, i.e. blackpaint.vmt[bmp] and blackpaint_trim.vmt[bmp] and and i only wanted to change one of them, then using the above is fine.
but if i wanted to change both textures in unison, then the block of text to add to the .qc would have looked like this;

$texturegroup "skins"
{
{ "blackpaint.vmt" "blackpaint_trim.vmt" }
{ "greenpaint.vmt" "greenpaint_trim.vmt" }
{ "rustpaint.vmt" "rustpaint_trim.vmt" }
}
you can see what happened there.
i hate being longwinded with explainations, because sometimes it just serves to confuse, so i'll leave it at that for now.

btw, if you don't use the correct spacing and "" marks exactly as i've done here, then it won't work.

[standard disclaimer applies ]